1. Field of the Invention
The present invention relates to an improved AC output power control system using solid state switches such as an inverter or a cyclo-converter wherein two groups of solid state switches are selected from at least three or three groups of solid state switches are alternatively turned on and off and the two groups are selected in predetermined order for the periods whereby low order harmonics are decreased and a stepless variable frequency control is attained.
The present invention relates to an improved AC output power control system using solid state switches such as an inverter or a cyclo-converter wherein the output current in the overlapping period is controlled and the current increase and decrease rate is controlled so as to decrease the low order harmonics.
2. Description of the Prior Art
FIGS. 1(a), (b), (c) are diagrams of typical embodiments of the AC output circuits using solid state switches 1 wherein solid state switches 1u - 1w or 1up - 1wn are operationally connected between a load 100 and a DC power source 200.sub.1 or an AC power source 200.sub.2.
FIG. 1a shows a three-phase half-wave inverter; FIG. 1b shows a three-phase bridge type inverter; FIG. 1c shows a three-phase half-wave cyclo-converter.
Taking the embodiment of FIG. 1a, in the conventional system, the switches 1u - 1w are respectively turn-on controlled as shown in FIG. 2. In the conventional turn-on control for the drive of an AC motor, a rotation of the magnetic field in the low speed and low frequency output region is in step state and the torque pulsation is high and a smooth rotation is difficult to attain. In the constant frequency AC output feed, with a conventional control system, the low order harmonics are high and the required capacity for a filter is high. In synchronous motor drive such as a stepping motor, there is a step rotation and it is difficult to provide middle stop points. On the other hand, it has been known to use a control system for modulating based on a standard sine wave or on a sine wave approximate step wave. However, the control device is disadvantageously complicated. In the case of an inverter, the commutation frequency (switching frequency) is increased so as to increase the loss and the switching elements and auxiliary devices are expensively complicated. In the case of a thyristor invertor, it has been necessary to employ an expensive extinction system for high frequency such as a one using a commutation auxiliary thyristor. It has been impossible to employ a simple interphase commutation system shown in FIGS. 6a, 6b, 6c, because of the principle of modulation. The simple extinction system for low frequency shown in FIG. 6d has not been used in practice because of the increase of loss. The conventional variable frequency drive system can not be applied for fine control of a numeral control apparatus, etc. A low inertia DC motor for this purpose is expensive and, especially, a direct driving low speed-high torque DC servomotor.